In general, zeolites are materials that have been used in a wide range of applications, including catalysis, separation, water softening, and adsorption. The rigid molecular-sized pore structures in zeolites make them appropriate for separating gas molecules via the recognition of the minute difference in their shapes and/or sizes.
The molecular sieve characteristics of such zeolite have the potential to selectively separate CO2 from mixtures comprising CO2/N2/H2O, which results from post-combustion processes, and natural gas mixtures comprising CO2/CH4/H2O.
Furthermore, the pore size of 8-membered rings (8-MRs) in zeolite is greater than CO2 and similar to or smaller than N2 or CH4, and thus is suitable for CO2 separation. In particular, the molecular sizes of CO2, N2 and CH4 are 0.33 nm, 0.364 nm and 0.38 nm, respectively, whereas the greatest size of the 8-membered rings is about 0.43 nm (D. W. Breck, Zeolite molecular sieves: structure, chemistry and use. 1974: John Wiley & Sons, Inc.).
Multiple 8-membered ring zeolite membranes were designed in an attempt to capture CO2 from mixtures. Particularly, 8-membered zeolite and zeolite-like membranes such as DDR, SSZ-13 (CHA type) and SAPO-34 (CHA type) membranes are known to exhibit high performance for CO2 separation. Among the 8-membered ring zeolites, the pore size of Si-CHA zeolite (M. J. Diaz-Cabanas, P. A. Barrett, M. A. Camblor, Chem. Commun. 1998, Issue 17, 1881-1882) consisting only of silica is about 0.370 nm×0.417 nm, which enables separation of CO2 and N2 by their size difference. Although the separation of CO2 from N2 can be achieved by size exclusion using 8-membered ring pore apertures of Si-CHA, the separation of CO2 from H2O is more challenging due to the smaller molecular size of H2O (0.265 nm). In order to minimize H2O flux through CHA zeolite, hydrophilic properties, presumably due to Al constituents in CHA frameworks should be minimized, thus preferably requiring all-silica constituents (J. Choi, S. Ghosh, Z. P. Lai, M. Tsapatsis, Angew. Chem., Int. Ed. 2006, 45, 1154-1158; Korean Patent Nos. 10-1499905 and 10-1460322).
To date, there have been very insufficient efforts to develop methods for fabricating hydrophobic silica zeolite membranes that have 8-MRs or similar porous structures and that have a high potential for CO2/N2 separation even in the presence of water. Rather, development has been performed to ensure CO2/N2 separation potential based on the high ability of CO2 to be adsorbed into hydrophilic zeolite structures, but there is a problem in that, because the interaction between CO2 and zeolite structures is reduced at high temperatures, CO2/N2 separation is possible only at low temperatures. When water is present in a feed that is supplied to zeolite membranes, there is a problem in that the competitive adsorption of CO2 with water is reduced, and thus the preferential adsorption potential of CO2 is reduced so that the CO2/N2 separation potential is reduced.
Accordingly, the present inventors have made extensive efforts to overcome the above-described problems occurring in the prior art, and as a result, have found that, when chemical vapor deposition is applied to silica chabazite zeolite particles to remove defects from a chabazite zeolite membrane and to effectively control the pore size of the membrane, it is possible to produce a chabazite zeolite membrane with a controlled pore size, which has enhanced CO2/N2 separation performance even not only under dry conditions, but also under moist conditions, thereby completing the present invention.